Left panel: comparison between N_tot (HCNH⁺) and N_tot (HCO⁺). The latter was computed from N_tot (H¹³ CO⁺) taken from Fontani et al. (2018) by correcting it for the ¹²C/¹³C ratio obtained at the Galactocentric distance of the sources from the trend of Yan et al. (2019). Middle panel: same as left panel, but for the abundances relative to H₂. As in the middle panel of Fig. 3, the purple rectangle and the cyan square show the predictions of the WM and CM (Sect. 5 and Fig. 7), respectively, that best reproduce the observed abundances of the three species HCNH⁺, HCN, and HCO⁺. Right panel: comparison between the line FWHM of HCNH⁺, derived in this work from the J = 3−2 line, and H¹³CO⁺, derived in Fontani et al. (2018) from the J = 1−0 line at ~87 GHz. The symbols have the same meaning as those in the left and middle panels.